Extraction of hydrocarbons such as oil and gas from underwater wells involves use of various elongate and typically cylindrical members, such as the risers which conduct the hydrocarbons themselves to the surface. There are various situations in which it is necessary to mount a clamp upon such a member which can support an axial load and resist being moved along the member. For example, an umbilical or riser is often tethered to the seabed using a tether clamp secured to an end portion of the umbilical/riser. The tether itself leads from the tether clamp to a structure which is fixed to the seabed, such as a gravity base or suction pile.
Existing tether clamps typically comprise part-cylindrical and largely non-compliant clamp shells shaped to be arranged around the riser/umbilical and drawn together using threaded fasteners to secure the clamp in place.
One of the technical challenges in this context is that the riser or umbilical may contract and expand in service, especially since it may be subject to significant variations of temperature and internal pressure. Some degree of compliance thus needs to be built into the clamp. Typically in known tether clamps it is the fasteners used to draw the clamp shells together that contribute the greater part of this compliance, but the variation in diameter which can consequently be accommodated is small.
Other clamps for mounting upon risers (typically for locating buoyancy, to prevent it from moving along the length of the riser) sometimes utilise straps to secure part-annular clamp bodies against the riser. UK patent 2286649 (granted to Balmoral Group) and UK patent 2288205 (granted to CRP Group Limited) concern clamps of this general type. The straps are formed e.g. of titanium or of composite material and are loaded with a large tension through bolts used to draw their ends together, by virtue of which the straps provide some compliance. Published US patent application 2007/0087639, filed in the names of Oram, Stokes and Reid, concerns another type of band clamp having a radially resilient member. In one embodiment this is an elastomeric body between the band and the clamp shells beneath. In another, a set of arcuate resilient leaves is provided within the clamp body and it is through these that the clamp seats upon the riser.